The invention relates generally to electronically commutated brushless motors, such as switched reluctance motors, high frequency induction motors, brushless AC motors, and brushless DC motors. More particularly, the invention relates to an electronically commutated brushless motor design and assembly process that provides a robust brushless motor capable of meeting the unique functional requirements in various applications, such as portable table saws, miter saws, site saws, and TGS-type combination saws.
Prior art electronically commutated brushless motors suffer from various limitations. One limitation is a restriction of airflow through the motor. In a typical universal motor housing, air is drawn in through vents in an end cap, passes over a brush gear assembly and windings, through a fan and exhausts out the other end of the motor.
In an electronically commutated brushless motor, air is also drawn in through an end cap but first must pass around the periphery of an electronics control module, installed at one axial end of the housing, before the air can pass through the rest of the motor. Thus, the electronics control module, which includes a potting boat holding an encapsulated printed circuit board (PCB), impedes the airflow by causing the air to first pass around the electronics control module. After passing around the electronics control module the air passes down through channels created by extruded fins of aluminum heat sinks, thereby cooling electronic components attached to the heat sinks. The air then continues over stator windings, passes through and around the stator, through a fan and exhausts out through the end of the motor. Thus, in electronically commutated brushless motors the electronics control module restricts the airflow through the motor.
The obstruction to airflow in electronically commutated brushless motors is further compounded by the housing molding process. To effectively mold and produce the housing, it must have draft added on both its internal (core) and external (cavity) sides. Since the geometry at the mouth of the housing is fixed by mounting interface requirements with existing products, the draft closes (i.e., narrows) the housing down about the electronics control module, thereby further restricting the airflow around the electronics control module and through the motor.
A second limitation of known electronically commutated brushless motors is that the motor is typically longer than a typical universal motor. Due to the longer motor, electronically commutated brushless motors are difficult to utilize in many power tools where it is desirable to keep the overall axial length of the motor, or housing, as short as possible. This is especially true with saws, such as miter saws and other saws, because when the saw (and the motor coupled to the saw) is tilted at an angle, an extra long motor housing can cause interference with a fence or the table of the saw. For example, in a TGS-type combination saw, the axial length of the motor housing must be short enough so that it does not protrude beyond the frame of the saw. If it does it will prevent the table from being flipped over. As another example, the axial length of the motor in a small portable table saw should be such that when attached to the saw gear case, the motor housing should fit inside the skirt that forms the base of the table. As yet another example, in a miter saw, where the bevel and miter functions will tip the end of the motor towards the table, the axial length of the motor should be such that the motor does not contact the table fence.
A third limitation of known electronically commutated brushless motors is the inability to insure proper alignment of the registering means on the rotor shaft with the rotor pole, and the position sensor with the stator during the assembly process. The registering means could be any suitable registering means such as an interrupter or a magnet, and the position sensor could any suitable sensor such as an optical sensor or a Hall Effect sensor. In typical electronically commutated brushless motors, the position of the registering means, relative to the position sensor, determines the position of the rotor, relative to the stator. In electronically commutated brushless motors it is critically important to know the exact position of the rotor when the electronic switching signals, which switch the direction of the flux in the motor winding(s), are provided by an electronic controller. If the alignment of the registering means with the rotor pole is off, or the alignment of the position sensor with the stator is not precisely set, then the position of the registering means, as detected by the position sensor, will provide an inaccurate indication of the position of the rotor, relative to the stator. If the position of the rotor is not accurately determined, the electronic switching motor will very quickly lose power and torque.
A fourth limitation of known electronically commutated brushless motors is meeting the requirements for double insulated construction as described by Underwriters Laboratories (UL) and other compliance agencies. Double insulated motor designs, which eliminate the need for a ground wire in the power cord, have been implemented on universal motors. This is a preferred construction for hand held and table mounted power tools since the alternative, grounded tools, rely on there being a solid ground connection available on a job site, which often is not the case. The basic requirement is that the design must provide at least two levels of insulation between live components, such as the windings, and any metallic components, such as the shaft or screws, that are accessible to the user. Known electronically commutated brushless motors do not implement a double insulated construction design.
Therefore, it would be desirable to provide an electronically commutated brushless motor design that provides increased airflow through the motor. It would further be desirable provide an electronically commutated brushless motor having an overall axial length suitable for applications requiring a shorter motor. Even further, it would be desirable to provide an electronically commutated brushless motor design that insures accurate alignment of the optical encoder with the rotor poles, and accurate alignment of the optical sensor with the stator during assembly of the motor. Further yet, it would be desirable to provide an electronically commutated brushless motor that implements a double insulated design without significantly increasing the cost or complicating the manufacturability and/or assembly of the overall motor.
The present invention relates to an electronically commutated brushless motor design, which overcomes the various drawbacks described above. In a preferred form, the invention relates to brushless AC motor, although it will be appreciated that the invention is also applicable to brushless DC motors.
In one aspect of the present invention a housing of the motor is provided with a draft angle that increases the airflow through the housing to allow more efficiently cooling of the motor.
In a second aspect of the present invention the electronically commutated brushless motor design provides a capacitor mounting arrangement that allows the overall axial length of the motor to be made shorter. The capacitors are mounted on a circuit board, which is adapted to slide into a housing protrusion, or bulge, formed on the sidewall of the housing rather than at one axial end of the housing. Thus, the overall axial length of the housing is made shorter, thereby allowing a wider range of applications for the motor in which the motor must be situated into different positions without interfering with other components of its associated tool.
In a third aspect of the present invention the electronically commutated brushless motor provides a housing that allows accurate alignment of a stator and a position sensor, such as an optical sensor, relative to each other. This is accomplished by using a housing molding core that produces a housing that includes both a bridge on which the position sensor is mounted, and stator locating ribs. Typically, the molding core for the housing forms locating ribs on an interior surface of the housing, which are used to precisely align the stator when it is inserted into the housing, but does not include a mounting bridge for the position sensor. By molding the housing to include both a position sensor mounting bridge and the stator locating ribs, variances in the positioning of the position sensor, relative to the stator, are avoided. Therefore, the position sensor and the stator will be accurately aligned when installed, without the need for time consuming alignment procedures, or tests, during the assembly of the motor.
In a fourth aspect of the present invention the electronically commutated brushless motor incorporates a double insulation (DI) feature, thereby eliminating the need for a direct ground cable in the power cord. The DI design includes insulating strips between the stator and stator windings, and an insulating sleeve disposed between the rotor shaft and the rotor laminations. Therefore, there are two layers of insulation between metal parts accessible to a user and parts of the motor in which electrical current flows. Alternatively, the motor housing, which supports the stator, is also constructed of a non-conductive material.